When weaving on conventional weaving machines using weaving shuttles, the dobbies operate in such a manner that the shafts are continuously in motion and the upwardly and downwardly moved shafts carry out a so-called symmetrical shed movement. In this shed movement, the upper and lower sheds move symmetrically in relation to the central position of the warp threads, that is to say, move towards and away from one another by equal distances at each instant of time.
In cases where non-symmetrical shed movement is required, hitherto weaving shafts driven by eccentric discs have been used, the desired weaving shaft movements being formed by suitable shaping of the eccentric outline. When using programme-controled dobbies, for example of the Hattersley type, wherein plates or hooks operated by means of lifting blades are used, it is per se not possible to form a non-symmetrical weaving shaft or shed movement.
An arrangement which could be considered would be to give the forwardly and rearwardly moved lifting blades non-identical speeds. But this kind of operation involves difficulties in the case of dobby apparatus known at the present day. Per se it is also possible to make the lifting blades come into action later when the shafts are raised than when the shafts are lowered, for example. But this has the result that those shafts which are to remain in the raised position begin to descend owing to the earlier commencement of the down movement and remain stationary again only when the raising of the lowered shafts has begun, to move entirely into the raised position again only at the end of shed forming. This method of operation results in unnecessary movements and unsteadiness at the shedding and rockers, which is disadvantageous. Also, increased wear on the dobby occurs. A further solution which might be considered would be to make the duration of the open shed position as long as possible. Since this reduces the time available for shed changing, this requires extremely quick shed changing and this in turn involves the disadvantage of over-stressing the moving parts of the dobby.
The closest prior art known to the applicant in connection with the invention presented in this application for Letters Patent is in:
U.s. pat. No. 2,924,247 and PA1 U.s. pat. No. 2,955,619.